Computer Interfacing
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Galaxy intruder alarm RS485 voltages

Clarifiction of RS485 working voltages


       Computer Interfacing Forum Index -> RS422 and RS485
Author Message
Morph 81

Oct 20, 2008 8:36 pm

Please can someone help.

I have a Honeywell Galaxy Alarm system, it has a percentage monitor for the data but I would like a secondary test.

I have metered the two line and I have +0.6 volts.

From the Data sheet I can see that the min output voltage is +/-1.5v does this mean I have a fault with my outgoing data from the control panel?
Site Admin

Joined: 13 Mar 2007
Posts: 145
Location: Netherlands and Kazakhstan

Oct 21, 2008 3:10 am

No, you probably don't have an output voltage problem. As you can see on the page with electrical characteristics of RS485, the minimum input sensitivity of ports should be 200 mV. This means that every receiver should be able to decode the signals if they are 600 mV.

The actual output voltage of an RS485 driver may change based on the number of receivers on the bus, installed termination resistors etc. The 1.5 Volt is what the standard recommends, but manufacturers sometimes install driver chips with lower driver voltages to preserve energy, especially on battery operated devices.

As a side note, where did you measure the 0.6 Volt? You should measure between the two data lines (A,B or data+,data-) and not relative to the supply voltage ground. RS485 signals are differential and floating compared to ground to reduce noise.
Morph 81

Oct 21, 2008 6:35 pm

Lammert, thanks for the advice.

The 0.6v was between the AB lines.

If I have 0.6v at the panel should it be the same voltage all the way to the end of the line?

Ps I am using a 670 ohm resistor as an end of line.

Nov 20, 2009 11:11 pm

Please can anyone give me an answer?/
I have been asked to move a clients galaxy keypad as he is having work done and in turn damaged the rkp cable.
I have replaced cable, fuses, and even rkp, but the display still lights up with
'**********' flashing across????

Not sure of the next step if any.

Kind Regards for any answers Sad

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